Benzyl mercaptan is particularly useful as a raw material for the synthesis of herbicides in the thiocarbamate family. An excellent purity of benzyl mercaptan is required for this use.
The synthesis of benzyl mercaptan by the action of hydrogen sulphide on benzyl alcohol in the presence of a catalyst is not economically viable due to the high price of benzyl alcohol and its undesirable characteristics during catalysis (deactivation of catalysts and rapid loss of selectivity).
An economically more attractive synthetic route is the nucleophilic substitution of the chlorine atom in benzyl chloride by an alkaline sulfhydrate, ammonium sulfhydrate or an amine sulfhydrate. This reaction, generally carried out in an alcoholic medium, has long been known:
C. Marcker, Liebigs Ann. Chem. 136, 76 (1865)
U.S. Pat. No. 2,456,588 (1948)
H. Zinner, Chem. Ber. 86, 825 (1953)
J. E. Bittell et al, J. Org. Chem. 43, 1687 (1978)
U.S. Pat. No. 4,082,790 (1978)
but the yields obtained are modest and conversion is incomplete. These references are hereby incorporated by reference. According to U.S. Pat. No. 4,082,790, the use of butylamine sulfhydrate in a medium of isopropanol gives a mixture containing 10% of unconverted benzyl chloride, 82% of benzyl mercaptan and 6% of benzyl sulphide. The use of ammonium sulfhydrate in a methanol medium gives a mixture containing 92% of benzyl mercaptan and 8% of benzyl sulphide.
Previous methods do not enable a benzyl mercaptan to be economically obtained meeting the specifications required by users, in particular purity greater than 98% and benzyl chloride content less than 0.1%. Indeed, the separation of benzyl chloride by distillation from crude benzyl mercaptan is very difficult. On the other hand, the formation of byproducts (benzyl sulphide and incidentally benzyl disulphide), although separable by distillation, is prejudicial to the economics of the process. Finally, the use of an alcohol solvent, which has to be recycled or destroyed before being discarded, complicates a production plant.
Thus, there is a need for a process for producing benzyl mercaptan which provides at the same time substantially total conversion of the benzyl chloride and good selectivity for benzyl mercaptan. This is without the use of alcohol as the reaction medium.